Oxidation of toluate esters and xylene mixtures



United States Patent OXIDATION OF TOLUATE ESTERS AND XYLENE MIXTURESIrving E. Levine, Albany, and William G. Toland, In, San Rafael, Calif.,assignors to California Research Corporation, San Francisco, Calif., acorporation of Delaware N 0 Drawing. Application August 26, 1952, SerialNo. 306,497

3 Claims. (Cl. 260-475) This invention relates to a processfor producingmonoesters of phthalic acids and saturated aliphatic alcohols containing1 to 4 carbon atoms per molecule.

Mono-esters of phthalic acids and saturated aliphatic alcoholscontaining from i to 4 carbon atoms per molecule can be produced byoxidizing esters of toluic acids and saturated aliphatic alcoholscontaining from 1 to 4 carbon atoms per molecule with air at about 400F. in the presence or absence of catalysts. The yields of phthalic acidmono-esters thus obtained are ordinarily in the range from 30 to 70 molepercent based on toluic acid esters converted. That higher yields arenot obtained is probably due to the fact that rather high temperaturesand long contact times are required to oxidize the toluic acid esters tophthalic acid mono-esters.

It has now been found that by oxidizing a liquid mixture of an alkylbenzene hydrocarbon, preferably xylene, and esters of toluic acids withsaturated aliphatic alcohols containing 1 to 4 carbon atoms with a freeoxygen-containing gas in the presence or absence of catalysts,considerably higher yields of phthalic acid mono-esters are obtained andthat the oxidation of the toluic acid esters to phthalic acidmono-esters proceeds at a satisfactory rate at substantially lowertemperatures. This oxidation can be performed in a continuous process inwhich a free oxygen-containing gas is passed through a liquid mixture ofxylene and an ester of toluic acid with a saturated aliphatic alcoholcontaining 1 to 4 carbon atoms at a temperature in the range from 200 F.to 400 F., and preferably in the range from 220 F. to 375 F., in anoxidation zone, fractionally distilling the reaction product to separatean overhead fraction comprising unreacted xylene, unreacted methyltoluate and toluic acid and a bottoms fraction comprising half-esters ofphthalic acid, esterifying the toluic acid of the overhead fraction witha saturated aliphatic alcohol containing 1 to 4 carbon atoms and thenreturning the overhead fraction including the esterified toluic acid tothe oxidation zone with further quantities of xylene. Alternatively, theentire crude oxidation product can be contacted with an alcoholcontaining 1 to 4 carbon atoms under esterifying conditions to esterifythe phthalic acid mono-ester and the toluic acid contained therein. Theesterification product is then distilled to separate xylene and toluicacid ester as an overhead fraction and phthalic acid diesters as thebottoms fraction. The overhead is returned to the oxidation zone withfurther quantities of xylene. When it is desired to operate continuouslyand produce only phthalic acid mono-esters as the end product of thereaction, a mole ratio of toluic acid ester to xylene of at least 1:1and in the range 1:1 to 5:1 is maintained in the oxidation zone.

The advantages of the process of the invention are illustrated by thefollowing examples.

Example 1 500 g. of mixed xylenes containing 61.8% meta-xylene, 30.6%para-xylene, 4% ortho-xylene and 3.6% of paraffins boiling in the xylenerange and 500 g. of mixed methyl ice 2 toluates containing85% methylmeta-toluate and methyl para-toluate and 2 g. of cobalt naphthenate containing 6% by weight of cobalt were introduced into an oxidationreactor. The reactor consisted of 6 feet of stainless steel pipe mountedvertically and encased in an electrically heated jacket. Temperaturecontrol is obtained by controlling the pressure applied to the boilingwater in the jacket. The overall volume of the oxidation reactor was 4liters. High pressure air metered through a vrotameter wasintroducedinto the base of the reactor 1 through a inch jet. A refluxcondenser provided with a water separator was mounted above theoxidation reactor. Tail gases from the condenser passed through adrop-out pot to a Grove pressure regulator and then to a vent. Thereactor was heated to 325 F. and maintained under a 50 p. s. i. g.pressure. Air was introduced at the rate of 10 cubic feet per hour.After a 10 minute induction period tail gas analysis indicated 0.2% ofcarbon dioxide and 14.2% of oxygen present. After 30 minutes operationthe tail gas contained 0.8% carbon dioxide and 8.4% oxygen. The effectof temperature was de termined by raising the temperature of the reactorto 340 F. and the pressure to p. s. i. g. while maintaining the air rateat 10 cubic feet per hour. Under these conditions tail gas analysisshowed 1.4% of carbon dioxide and 8.5% of oxygen. These conditions weremaintained for a period of 35 minutes, at which time the temperature wasdropped to 300 F. and the pressure was reduced to 50 p. s. i. g. Underthe latter conditions tail gas contained 1% carbon dioxide and 13% ofoxygen. These conditions were maintained until the end of the run, whichhad a total duration of minutes. Products were drained from the reactorand water separator and consisted of 30 cc. of a water layer and 993.2g. of an organic layer. The products were worked up by steam strippingto remove unreacted xylene and part of the unreacted ester. The strippedreaction product was then fractionally distilled to remove the remainingxylene and methyl toluate. Phthalic acid was separated by filtering thedistillation bottoms. Chloroform extraction was employed to remove theremaining products which consisted of toluic acid, methyl toluate andmethyl acid phthalate from the bottoms. Yields were as follows: 8.8% ofphthalic acid based on the weight of xylene converted; 73.6% toluic acidbased on the weight of xylene con verted; and 109% by weight of methylacid phthalate based on the weight of methyl toluate converted. Highmolecular weight side reaction products amounted to less than 1% of thecharge to the reactor. Some xylene was lost with the tail gases.

Example 2 Mixed methyl toluates alone were oxidized in the mannerdescribed in Example 1. The cobalt naphthenate was again employed as acatalyst, the amount introduced being suflicient to give the reactionmixture a cobalt content of 0.05% by weight. Air was blown through themethyl toluates for a period of 3.2 hours, during which the temperatureof the oxidation reactor was maintained at 400 F. 1.21 moles of methyltoluate were charged to the reactor and the oxygen content of the airpassed into the reactor was 1.67 moles. The reaction product was workedup as in Example 1, except that steam stripping was omitted and theyield of phthalic acid mono-esters produced was 54.4% based on theweight of methyl toluate converted.

From the above examples it is clear that toluate esters can be oxidizedto phthalic acid mono-esters at lower temperatures and at higher yieldsif the toluic acid esters are subjected to oxidation in the presence ofsubstantial amounts of xylene.

Similar comparative yields are obtained when ethyl toluate, isopropyltoluate and butyl toluate are oxidized with air alone and in thepresence of xylenes.

Similar comparative yields are also obtained when the oxidations areconducted in the absence of a catalyst, although the rates of oxidationare appreciably lower. If it is desired to employ a catalyst in thereaction, oil-soluble compounds of metals of groups VII and VIII of theperiodic table are useful catalysts.

No solvent need be employed in the work-up of the reaction product aswas done in Example 1. Satisfactory separation can be obtained byfractionally distilling the reaction product to remove unreacted xylene,filtering the bottoms fraction to remove phthalic acid, and thenfractionally distilling to separate toluic acid and unreacted methyltoluate from the methyl acid phthalate. The toluic acid is thenesterified and returned to the reaction zone together with unreactedmethyl toluate, unreacted xylene and incremental xylene.

The reaction as indicated above is carried out at a superatmosphericpressure sufiicient to maintain the xylene in liquid phase. Pressuresranging from 50 to 300 p. s. i. g. are satisfactory for this purpose.Xylene vapors are readily recovered from the tail gas for return to thereaction zone.

We claim:

1. An oxidation process which comprises forming a liquid mixture of 'axylene and an ester of toluic acid with an alkanol containing 1 to 4carbon atoms and passing a free oxygen-containing gas through the liquidmixture at a temperature in the range from 200 F. to 400 F.,

4 the mol ratio of toluic acid ester to xylene in the liquid mixturebeing in the range 1:5 to 5:1.

2. The method as described in claim 1, wherein the oxidation isconducted in the presence of an oil-soluble compound of the metalsselected from groups VII and VIII of the periodic table.

3. A process for producing mono-esters of phthalic acids with loweraliphatic alcohols containing 1 to 4 carbon atoms which comprisespassing a free oxygen-containing gas through a liquid mixture of axylene and methyl toluate at a temperature in the range from 220 F. to375 F. in an oxidation zone, the mole ratio of said methyl toluate toxylene being in the range of 1:1 to 5:1, fractionally distilling thereaction product to separate an overhead fraction comprising unreactedxylene, unreacted methyl toluate and substantially all of the toluicacid contained therein and a bottoms fraction comprising a phthalic acidmono-ester, esterifying the toluic acid of the overhead fraction withmethanol and then returning the overhead fraction including theesterified toluic acid to the oxidation zone with further quantities ofxylene.

References Cited in the file of this patent UNITED STATES PATENTS

1. AN OXIDATION PROCESS WHICH COMPRISES FORMING A LIQUID MIXTURE OF AXYLENE AND AN ESTER OF TOLUIC ACID WITH AN ALKANOL CONTAINING 1 TO 4CARBON ATOMS AND PASSING A FREE OXYGEN-CONTAINING GAS THROUGH THE LIQUIDMIXTURE AT A TEMPERATURE IN THE RANGE FROM 200* F. TO 400* F., THE MOLRATIO OF TOLNIC ACID ESTER TO XYLENE IN THE LIQUID MIXTURE BEING IN THERANGE 1:5 TO 5:1.